1. Field of the Invention
This invention relates to testing telephone subscriber loops and, more particularly, to ballistic testing of subscriber ringers connected to extremely long subscriber loops.
2. Description of the Prior Art
The conventional procedure for sensing the presence or absence of bridged ringers at the end of a telephone customer loop is by securing an indication proportional to the total line capacitance. A telephone ringer presents a substantial capacitance across the telephone line and, not only can the presence of such a ringer be detected by a capacitance measurement, but the number of such ringers on any telephone line can similarly be detected.
A measure of the total line capacitance is obtained by alternately charging and discharging the telephone line from a test battery and noting the ballistic deflection of a milliammeter measuring the resulting currents. On normal telephone lines, this procedure is adequate to detect the presence of a ringer on the line and, in many cases, to ascertain the number of ringers bridged on the particular line.
The sensitivity of the ballistic ringer test decreases with increasing loop length due to the increase in capacitance associated with the telephone line itself. It is estimated that the limit for accurately detecting the presence of a telephone ringer on the line using this method is about twenty miles. This limitation is of no concern on telephone loops in urban and suburban areas since the lengths of such loops fall well within this limit.
On longer rural loops, however, and particularly those which require some form of carrier system to transmit voice signals, the limit is often exceeded and reliable ringer continuity tests using the ballistic procedure cannot be accomplished. Alternate means for testing ringers in loops of great length involve expensive remote testing devices in which the information controlling the tests and reporting the test results must be transmitted between the central office location and the remote testing location by alternating current tones. One such tone-controlled testing arrangement is disclosed in C. R. Davies U.S. Pat. No. 3,410,966, granted Nov. 12, 1968. Tone-controlled testing arrangements, however, are expensive, bulky, and complex, leading to higher costs and lower reliability.